The invention relates to an optical storage apparatus for recording and reproducing information by using a laser beam and to a recording and reproducing method of an optical storage medium. More particularly, the invention relates to an optical storage apparatus for recording and reproducing data at a density that is smaller than a beam diameter as is known as an MSR (Magnetically induced Super Resolution) technique and to a recording and reproducing method of an optical storage medium.
In recent years, an optical disk is being highlighted as an external storage medium of a computer. In the optical disk, by forming magnetic recording pits of a submicrometer order onto a medium by using a laser beam, a recording capacity can be remarkably increased as compared with a floppy disk or a hard disk as a conventional external storage medium. Further, information can be rewritten in a magnetooptic disk as a perpendicular magnetic storage medium using a rare earthxe2x80x94transition metal system material, so that its future development is more and more expected.
The optical disk of, for example, 3.5 inches has a storage capacity of 540 MB or 640 MB on one side. This means that a storage capacity of one floppy disk of 3.5 inches is equal to about 1 MB and one optical disk has a storage capacity as much as that of 540 or 640 floppy disks. As mentioned above, the optical disk is a rewritable storage medium having an extremely high recording density. However, in order to get ready for the multimedia age in the future, it is necessary to increase the recording density of the optical disk even higher than that of the present optical disks. To increase the recording density, a greater number of pits have to be recorded on the medium. For this purpose, it is necessary to further decrease the pit size from the present size and to narrow the interval between the pits. In case of raising the recording density by such a method, it is further necessary to shorten a wavelength of the laser beam below the present wavelength of 670 nm. However, in case of considering the practical use, the pit size has to be reduced in the present wavelength of 670 nm. In this case, as for the recording, a pit which is smaller than the beam diameter can be formed by controlling a power of the laser beam. As for the reproduction, however, when the pit that is smaller than the beam diameter is reproduced, a crosstalk with the adjacent pit increases. In the worst case, the adjacent pit also enters the reproducing beam. When a practical use is considered, it is very difficult to use such a method.
As a method of reproducing a pit smaller than the beam diameter by the existing wavelength of 670 nm, there is a magnetooptical recording and reproducing method represented by JP-A-3-93058 and this method is known as a recording and reproducing method by the MSR (Magnetically induced Super Resolution). Presently there are two general methods of MRS an FAD (Front Aperture Detection) system and an RAD (Rear Aperture Detection) system. In the FAD system, as shown in FIGS. 1A and 1B, a recording medium is divided into a recording layer 220 and a reproducing layer 216. Data is reproduced using a reproducing magnetic field Hr applied to the medium in a state in which a laser spot 222 of a reading beam is irradiated, in this instance. In this instance, as depending on a temperature distribution within heating by the laser spot 222, a magnetic coupling of a switching layer 218 formed in a boundary with the recording layer 220 is released, such and such a portion of the beam spot is influenced by the reproducing magnetic field Hr and becomes a mask. On the other hand, as for a portion of the next recording pit, the magnetic coupling in the switching layer 218 is held and this portion becomes an opening 224. Consequently, within the laser spot 222, only a pit 230 of the opening 224 can be read without being influenced by the adjacent pit 226. On the other hand, according to the RAD system, as shown in FIGS. 2A and 2B, an initialization to align a magnetizing direction of the reproducing layer 216 to a predetermined direction is performed by using an initializing magnet 232. The reading operation is performed by slightly raising a reproducing laser power upon reproduction. Depending on the temperature distribution within laser spot 234 of the reading beam, a mask 236 in which initial magnetization information remains and an opening 238 in which the initial magnetization information is erased and magnetization information of the recording layer 220 is transferred are formed in the reproducing layer 216. The magnetization information of the recording layer 220 transferred to the reproducing layer 216 is converted into an optical signal by a magnetooptic effect (Kerr effect or Faraday effect), so that data is reproduced. In this instance, as compared with a pit 228 of the recording layer 220 which is being read out at present, the pit 230 of the recording layer 220 to be subsequently read out is not transferred due to the formation of the mask 236 by the initial magnetization information in the reproducing layer 216. Therefore, even when the recording pit is smaller than the laser spot 234, no crosstalk occurs and the pit which is smaller than the beam diameter can be reproduced. Further, by using such a magnetically induced super resolution, since an area of the recording layer 220 except for the reproduced portion is masked by the initialized reproducing layer 216, a pit interference from the adjacent pit doesn""t occur. Further, since a pit interval can be reduced and a crosstalk from the adjacent track can be also suppressed, a track pitch can be reduced and a high density can be realized even by using the existing wavelength of 780 nm.
However, in the conventional optical disk apparatus using such a magnetically induced super resolution, there is a problem that a proper reproducing operation cannot be performed unless an intensity of the reproducing magnetic field which is used upon reproduction is strictly controlled. The reason is that, for example, when the reproducing magnetic field Hr is too low in the FAD system in FIG. 1A, a forming range of the mask 226 in FIG. 1B by the magnetization of the reproducing layer 216 is reduced and the pit 228 is not masked, so that a crosstalk occurs. When the reproducing magnetic field is too strong, the forming range of the mask 226 is widened and the pit 230 is also partially masked, so that a reproducing level decreases and an error occurs. At the same time, the reproducing magnetic field Hr also acts on the recording layer 220 and there is a possibility of erasure of the recording data. When the initializing magnetic field is too low in the RAD system in FIG. 2A, an erasing range by a beam heating of the initializing magnetization of the reproducing layer 216 is widened and the forming range of the mask portion decreases, so that the pit 230 in FIG. 2B is not masked and a crosstalk occurs. When the initializing magnetic field is too strong, the erasing range by the beam heating of the initializing magnetization of the reproducing layer 216 is narrowed and the forming range of the mask 236 is widened, so that the pit 228 is partially masked, the reproducing level decreases, and an error occurs. At the same time, when the initializing magnetic field is too strong, the magnetic field also acts on the recording layer 220 and there is a possibility of erasure of the recording data. Moreover, the reproducing magnetic field and the initializing magnetic field are dependent upon the an environmental temperature of the apparatus. That is, when the environmental temperature in the apparatus changes to the lower side, hysteresis characteristics of the reproducing layer become thick. In order to obtain the same magnetizing characteristics (magnetic flux density), the reproducing magnetic field has to be intensified. On the contrary, when the environmental temperature changes to the higher side, the hysteresis characteristics of the reproducing layer become thin. In order to obtain the same magnetizing characteristics, the reproducing magnetic field has to be weakened.
According to the invention, there are provided optical storage apparatus and recording and reproducing method of an optical storage medium, in which when a magnetically induced super resolution is used, an intensity of an external magnetic field to be used upon reproduction is properly set, thereby preventing the signal level of a reproduction signal from deteriorations and ensuring that reproduction can be performed.
First, an optical disk apparatus of the invention uses a magnetooptic storage medium having at least a recording layer to record data and a reproducing layer to reproduce the data recorded on the recording layer on a substrate. A recording unit records data into the recording layer of the magnetooptic storage medium at a recording density that is smaller than a beam diameter of a laser beam. A reproducing unit reproduces the data recorded in the recording layer of the magnetooptic storage medium at a recording density smaller than the beam diameter by setting a reproducing magnetic field and a reproducing laser power which are applied by a magnetic field applying unit such as permanent magnet, electromagnet, or the like to proper values. In addition to those units, a reproducing magnetic field correcting unit is provided, a reproducing state by the reproducing unit is monitored while increasing the reproducing magnetic field by using a predetermined reproducing magnetic field as an initial value, and the reproducing magnetic field when the reproducing unit is in a reproducible state is decided as an optimum magnetic field. Therefore, even when the reproducing power and/or environmental temperature in the apparatus change or when a medium having different characteristics is loaded, a situation such that the reproducing magnetic field is too strong and a mask portion is widened, so that the recording data cannot be read out or the recording data is erased can be certainly prevented. It is also possible to reduce an electric power consumption of the apparatus by reducing a current to be supplied to the magnetic field applying unit. Further, a situation such that the reproducing magnetic field is too weak and the mask portion is narrowed, so that an error occurs by a crosstalk with an adjacent pit can be also certainly prevented.
The reproducing magnetic field correcting unit sets a reproducing magnetic field obtained by adding a predetermined value to a reproducing magnetic field in a reproducible state in which the number of times of dissidence of a reproduction data bit is equal to or less than a threshold value to an optimum reproducing magnetic field. The reason is as follows. When an external magnetic field is increased from an initial value, for example, a change in number of times of dissidence of the reproduction data has characteristics having a shoulder such that the number of times of dissidence decreases to a specified value or less and is stabilized and, after that, it again increases. Accordingly, since a shoulder portion in which the number of times of dissidence is stabilized to a threshold value or less is detected as a reproducible state, the predetermined value is added to the reproducing magnetic field in the reproducible state so that an optimum value is located to almost the center of the stable portion. In this case, even if a predetermined coefficient xcex1 (=1.x) exceeding 1 is multiplied to the reproducing magnetic field in the reproducible state, the same result will be derived. The reproducing magnetic field correcting unit starts the correction of the reproducing magnetic field from a low magnetic field obtained by subtracting a predetermined value from the reproducing magnetic field initial value. The above point also depends on shoulder characteristics when the reproducing magnetic field is increased. Since the initial value of the reproducing magnetic field is ordinarily set to the shoulder portion, by starting the correcting process from a magnetic field that is slightly lower than such an initial value, the shoulder portion is surely detected and the optimum magnetic field can be set. In this case as well, even if the correction of the reproducing magnetic field is started from a low magnetic field obtained by multiplying a predetermined coefficient xcex2 (=0.x) smaller than 1 to the reproducing magnetic field initial value, the same result will be derived. The reproducing magnetic field correcting unit limits a correction value of the reproducing magnetic field so as not to be equal to or larger than a predetermined value in order to prevent that the recording data is erased at the time of the correcting process. The reproducible state is discriminated by the reproducing magnetic field correcting unit on the basis of any one of the following discriminating conditions.
I. A point that a level of a peak detection signal of an RF signal reproduced from a medium return light by the reproducing unit is equal to or larger than a predetermined value is detected, thereby deciding that the reproduction is possible.
II. The reproduction data of the reproducing unit and the recording data at the reproducing position which has previously been known are compared on a bit unit basis and a point that the number of bit errors (number of times of dissidence) is equal to or less than a predetermined value is detected, thereby deciding that the reproduction is possible.
III. A point that the number of correction errors for the reproduction data of the reproducing unit is equal to or less than a predetermined value is detected, thereby deciding that the reproduction is possible.
The reproducing magnetic field correcting unit determines the optimum magnetic field every predetermined zone of the optical storage medium and stores it into a memory (reproducing magnetic field storage table). A reproducing magnetic field setting unit of the reproducing unit reads out the optimum reproducing magnetic field of a zone corresponding to a reproducing position of the optical storage medium from the memory and drives the magnetic field applying unit. In this instance, the optimum reproducing magnetic field corresponding to the reproducing position of the optical storage medium is obtained by a linear approximation of the optimum magnetic field of the zone read out from the memory, thereby driving the magnetic field applying unit. The reproducing magnetic field setting unit of the reproducing unit corrects the optimum reproducing magnetic field decided by the reproducing magnetic field correcting unit by the temperature in the apparatus upon reproduction, thereby driving the magnetic field applying unit. The reproducing unit generates the optimum reproducing magnetic field decided by the reproducing magnetic field correcting unit only for a reproducing period in a sector of the optical storage medium in which a reproduction gate signal is ON.
The reproducing magnetic field correcting unit performs the correcting process of the reproducing magnetic field at the following timings.
I. At the time of the initialization diagnosing process in association with a turn-on of a power source of the apparatus
II. When the optical storage medium is loaded into the apparatus
III. When a change in temperature in the apparatus is equal to or larger than a predetermined value
IV. When a predetermined correcting valid time elapses by monitoring the elapsed time after the preceding correction
V. When a reproduction error occurs and a retrying process is performed
VI. When the apparatus is started up in a factory
The reproducing magnetic field correcting unit temporarily stops the correction when an interrupting request is generated from an upper apparatus during the correction of the reproducing magnetic field and restarts the process from the interrupted portion after completion of the interrupting process.
According to the invention, there is also provided a recording and reproducing method of an optical storage medium, comprising the steps of:
recording data into a recording layer of an optical storage medium at a recording density that is smaller than a beam diameter of a laser beam by using the optical storage medium having at least a recording layer to record data and a reproducing layer to reproduce the data recorded in the recording layer on a substrate;
reproducing the data recorded in the recording layer of the optical storage medium at a recording density smaller than the beam diameter by setting a reproducing magnetic field and a reproducing laser power which are applied by a magnetic field applying unit to proper values; and
prior to the reproduction of the optical storage medium, executing a reproducing magnetic field correcting process for monitoring a reproducing state while increasing the reproducing magnetic field by using a predetermined reproducing magnetic field as an initial value and for determining the reproducing magnetic field in a reproducible state as an optimum reproducing magnetic field.
Details of the recording and reproducing method of the optical storage medium are substantially the same as those in the construction of the apparatus.
The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description with reference to drawings.